Wouldn't you simply do a division on closing mid for each bar? I don't use easylanguage either, but if you're getting data in the form of OHLC bars, you do need to be very careful about what you take from each bar, because spreads need to be calculated on contemporaneous quotes and too much info is lost in OHLC bars. It would be different if you received a full tick history.
With 1m bars you can calculate a 1m line graph using closing mid (since you know the prices are contemporaneous). Or, you could use the opening prices (again, contemporaneous).
I expect this is why the ratio is plotted as a line rather than bars, as the graphics don't download full tick history.